In the Lego online community, greeblies are the intricate details on the sides of spaceships and sci-fi MOCs that define the mechanics and workings of what the MOC is meant to be. They were famously used in the giant Star Destroyer Lego set a few years ago, and also in the more recent Death Star II set. Plates with rails are often used for greeblies because they have a protrusion that only sticks out half a stud without needing a jumper plate. (source: Brickspace.org)

any dirt, grime, grease, or dust left behind after it has already been cleaned.

After the floor is mopped you should sweep it again to get all the greeblies that the mop leaves behind.
Also if you don't rinse the dishes before you put them in the dish washer, there will be greeblies left on them.